Particle detectors are important in many applications including mass spectrometers, particle accelerators and electron microscopes. In all of these cases charged particles need to be detected. A detector is provided that transforms the energy, or the presence, of the particle into a form that can be amplified and the amplified form of the signal is then measured to infer the presence of the particle. In a mass spectrometer an ion beam is provided, the energy and current of which carries information about the material being analyzed. In an electron microscope a beam of electrons scans a sample. The beam interacts with the sample in different ways to produce signals that can be detected. In a transmission electron microscope this is achieved as follows, an electron beam passes straight through the sample and is partially absorbed by the sample. The part of the beam that is not absorbed is detected by a particle detector on the far side of the sample and contrasts between areas of different densities are shown. This requires a relatively high energy beam and has the disadvantage that it causes damage to the sample. A further disadvantage is that thin samples are required.
A second form of signal, used in a scanning electron microscope, comprises particles of the original beam that are reflected by the sample. These are known as backscattered electrons and tend to have energies similar to the primary beam, typically 50 eV and above. Backscattered electrons provide good material contrast information, allowing different materials to be distinguished.
A third form of signal, again taken advantage of in the scanning electron microscope, comprises secondary electrons. Secondary electrons are given off by the surface of the sample when struck by a primary electron, that is to say an electron of the original beam. Secondary electrons are of rather low energies, typically of 5 eV and below, certainly below 50 eV. Secondary electrons carry information about the topography of a surface.
There are three main types of particle detectors currently in use in the above-mentioned applications. One is known as a scintillator, the second is a solid state detector, sometimes known as a silicon detector, and the third is a plate. The plate comes in two forms, the first known as a micro-channel plate (MCP), and the second known as a micro-sphere plate (MSP). A scintillator comprises a glass, or other transparent substrate coated with phosphor, or such a transparent material impregnated with scintillating material. The scintillator gives off photons when struck by an electron or other charged particle. The photons pass through a light guide to a photomultiplier tube (PMT). The PMT generates an electronic current which is proportional to the number of photons that reach it and amplifies the electronic signal to generate an output signal strong enough to be passed to a visual display unit or memory or image processing device. Significant photon losses are encountered when transmitting photons from the scintillating body through the light guide to the photomultiplier.
A solid state detector generates electron hole pairs when struck by energetic electrons. These pairs are amplified by an electronic circuit to generate a signal strong enough to be passed to a visual display unit.
One of the problems with a scintillator is that a sufficiently large signal is produced from a single electron only if the electron has sufficient energy, typically 10 keV. Secondary electrons, which have nothing like this level of energy, must therefore be accelerated in order to be detected. Thus the front face of the scintillator is given a voltage level in the region of +10 kV. Because of this field strength it is necessary to position the detectors far from the sample in order to prevent beam aberrations or deflections. This in turn means that detection efficiency is reduced. The same applies to a solid state detector.
The photomultiplier tube, too, has to have a high electric field in order to operate, but at the same time the output signal should preferably be at ground potential so that it can easily be amplified and passed directly to the visual display unit. The only way to do this is to have the front plate of the photomultiplier tube at a strong negative voltage, for example -1.5 kV. The photons are of course immune to electric field so it does not matter that there is a strong negative electric field between the scintillator and the photomultiplier tube.
The MCP (or MSP) is a plate, typically in the region of half a millimeter in thickness and having microchannels extending through the plate. The radius of the microchannel in an MCP is typically 10 .mu.m and the channel is straight. In the MSP the microchannels twist their way between the two surfaces of the plate. A voltage of typically 1 kV is placed across the thickness of the plate. An electron from the sample impinges on the wall of the microchannel and causes more than one secondary electrons to emerge from the wall. These in turn collide with the wall at a further point and generate more secondary electrons. Provided that there is an appropriately sized field across the plate the result is a multiplication effect. A multiplication of one to ten thousand can typically be obtained from a single MCP plate. However to ensure a sufficient output signal it is generally necessary to stack two or three of these plates one above the other. Each plate in the stack must have a sufficient field strength to ensure that overall a multiplication in the range of a hundred thousand to ten million is achieved.
Again the problem arises that the output signal has to be at ground for the purposes of the visual display unit but the anode of the MCP (or MSP) is at 2 to 3 kV. The problem can be solved by using a capacitor as a buffer. However this works only for low currents because the MCP output is limited to around ten percent of its bias current. An alternative method of buffering is to convert the signal into photons and use a light guide. Either way the result is a device that is complex and is not compact. Compactness is very important as the detector has to fit inside the column of the electron microscope housing or between the exit of the beam from the column and the sample.
An advantage of the MCP (or MSP) based detector is that it is more sensitive to low energy electrons compared to either a scintillator or solid state detector. It therefore does not need to have a strong field at its input face and it can thus be sited much closer to the sample or to the beam path. This improves detection efficiency.